1. Field of the Invention
The present invention relates to a semiconductor device utilizing a semiconductor thin film and to techniques for fabricating such a semiconductor device and, more particularly, to TFTs (thin-film transistors) utilizing a crystalline film containing silicon.
The semiconductor device referred to herein embraces every kind of device that functions by making use of a semiconductor. That is, the semiconductor device includes electrooptical devices (such as liquid crystal displays) and electronic devices on which such electrooptical devices are packed, as well as active components such as TFTs and MOSFETs (IGFETs). To make a clear distinction between two kinds of semiconductor devices, they will be referred to as the semiconductor device and as the display device, respectively.
2. Description of the Prior Art
In recent years, techniques for fabricating semiconductor circuits with TFTs (thin-film transistors) formed on substrates have evolved rapidly. Especially, an active matrix display using a crystalline silicon film (polysilicon film) as a thin-film semiconductor and having a common substrate on which both peripheral circuits and a pixel matrix circuit are packed have reached a practical level.
Among various kinds of active matrix displays, the active matrix liquid crystal display (AMLCD) is being actively developed as a display device adapted for use in a notebook computer, projector, or other mobile device. According to the mode of operation, AMLCDs are classified into two major groups: transmissive LCD and reflective LCD.
Presently, high information content, bright liquid crystal displays are being urgently developed. Structures have been developed in which each pixel is as small as less than 30 μm in square to meet the XGA standard (1024×768 pixels) or the SXGA standard (1280×1024 pixels).
In the aforementioned AMLCD, a voltage applied to the liquid crystal layer changes the optical response characteristic, thus turning on and off light. An auxiliary capacitor is usually added to each pixel to compensate for leakage of electric charge held on the liquid crystal layer.
We have already disclosed a method of obtaining a crystalline silicon film by making use of a catalytic element (typified by nickel) for promoting crystallization, to form a crystalline film containing silicon (see Japanese Unexamined Patent Publication No. 130652/1995). In particular, regions crystallized by direct introduction of a catalytic element (hereinafter referred to as the vertically grown regions) and crystallized regions (hereinafter referred to as the laterally grown regions) around the regions in which the catalytic element has been introduced are formed.
However, almost all catalytic elements are metal elements and so if the catalytic elements are left after the crystallization, the reliability of the finished TFTs will be impaired.